Fix query and aggregation of role implications

* Collate roles and contexts to issue a single query for all
  RoleImplication objects
* Fix aggregation of roles and contexts to handle the case where a user
  has one role in multiple contexts
Fix query when no RoleImplications are needed
Remove `Ownable` from `Help`, `Option`, `Category` models
Merge pull request #554 from reciprocity/feature/fix-implications-query

Fixups for Role Implications and Ownable models